With the old flat rate snippet i was able to make it work so that a shipping option was preselected once the user enters a post code.
However with the new custom shipping endpoint (which works good) there is no such way. Any idea how to do that? Otherwise maybe create a ticket for that.
BTW: Are you interested in a Java Library for the shipping endpoint? Currently there is helper library for php and js, we are using java, so probably we will convert it for java and can then send it to you guys.